The invention relates to digital data recording/reproducing apparatus and method for recording/reproducing a bit stream of an MPEG2 signal onto/from a magnetic tape at variable speeds dependent upon the bit rate of the encoded bit stream.
Progress is being made in the development of digital VTRs which compress a digital video signal by DCT (Discrete Cosine Transform) and variable length encoding and record the signal onto a magnetic tape by rotary heads. In such digital VTRs, modes for recording a video signal of the NTSC system or the like (hereinafter, referred to as an SD mode) and for recording a video signal of the HDTV system (hereinafter, referred to as an HD mode) can be set. In the SD mode, the video signal is recorded at a rate of 25 Mbps. In the HD mode, the video signal is recorded at a rate of 50 Mbps. Techniques for recording a bit stream of an MPEG2 (Moving Picture Experts Group) signal with such a digital VTR are being studied. The transport packets of the MPEG2 signal can be a signal of a digital CATV or the like.
In MPEG2, a multiprogram function is provided which enables a plurality of programs to be transmitted, individual encoding streams are time divisionally multiplexed on a relatively short unit basis called a transport packet. The transport packet is set to a fixed length of 188 bytes. A header portion contains content identification information of the transport packet. A transport packet necessary to reproduce a target program is selected and decoded using such information.
FIGS. 23A-D show a construction of the transport packet. As shown in FIG. 23A, the transport packet is composed of a header and a payload, the latter stores data information. As shown in FIG. 23B, the header comprises: sync data of eight bytes; a transport error indicator indicative of the presence or absence of an error in the packet; a payload unit start indicator indicative of the start of a payload unit; a transport priority indicator indicative of the significance of the packet; packet identification data (PID) indicative of an individual stream of the packet; transport scramble control data indicative of the presence and the kind or the absence of a scramble of the payload; adaptation field control data indicative of the presence or absence of an adaptation field; a continuity counter to detect whether a part of the packet has been abandoned; and an adaptation field which can be used to insert additional information regarding the individual stream and a stuffing byte.
As shown in FIG. 23C, the adaptation field includes: adaptation field length data indicative of a length of the adaptation field; a discontinuity indicator indicating that the system clock is reset and that new contents are being obtained; a random access indicator indicative of an entry point of random access; a priority stream elementary indicator indicating that an important portion exists in such a payload; a flag; an optional field; and a stuffing byte.
As shown in FIG. 23D, the optional field includes: PCR data; OPCR data; a splice countdown; a transport private data length; transport private data; adaptation field extension length; a flag; and an optional field. The PCR data is a time stamp for setting and calibrating a value serving as a time reference in a MPEG decoding system. The system clock (27 Mhz) is reproduced from the PCR by a PLL (phase lock-loop) circuit and the time base information of the transport packets is held to set a system clock reference for timing the subsequent decoding process.
As shown in FIG. 24, to record a transport packet of the MPEG2 signal with a digital VTR, a program is selected from programs A, B, and C (for example, program A) which are sent in a time division multiplexed form. The selected program is a burst-like form structured on a packet unit basis. If the data rate of the multiprograms is equal to, for example, 30 Mbps and the rate of the selected program is equal to, for instance, 10 Mbps, the data rate is converted from 30 Mbps to 10 Mbps by a rate converting buffer to record the selected program.
As shown in FIG. 25A, the transport packet of the selected program is supplied from an input terminal 101 to a rate converting buffer 102. The rate converting buffer 102 reduces the data rate by a factor of three. Thus, the rate is reduced from 30 Mbps to 10 Mbps. The rate converted transport packet is outputted to an output terminal 103 and recorded by a digital VTR. For reproduction, the signal of the digital VTR is rate converted to increase the data rate by a factor of three, i.e., opposite to the rate reduction upon recording.
When the signal reproduced by the digital VTR is decoded by a set top box, the system clock (27 Mhz) is reconstructed from the PCR by the PLL. The system clock is used as a reference for the timing of the decoding process. It is, therefore, necessary to retain the time base information of the transport packet through the recording and reproduction of the signal. However, rate conversion changes the time base information of the input signal during recording. Therefore, upon reproducing, the time base cannot be returned to its original state.
In MPEG2, an I picture which was intra-picture encoded, a P picture which was forward direction prediction encoded and a B picture which was bi-direction prediction encoded are transmitted. During variable speed reproduction continuous picture data cannot be obtained, therefore, only data of the I picture is used in variable speed reproduction because data of the P and B pictures cannot be decoded.
However, when the transport packet is recorded by the VTR, transport packets including the I picture are not sufficiently read in variable speed reproduction because the position of the recorded I picture is uncertain. Therefore, data of the I picture corresponding to a specific portion of a frame is not read at the correct time during variable speed reproduction and the picture quality deteriorates.
According to a method proposed by the inventor of the present invention, before entering the rate converting buffer 102, time information from a certain reference clock (indicative of the time at which the transport packet arrives) is added to each packet of the selected program received at an input terminal 104 by a time information adding circuit 105, shown in FIG. 25B. When the same reference clock is used for reproduction as for recording, the transport packet is transmitted according to the reference clock time information and the same time state as upon recording can be reconstructed during variable speed reproduction. The head sync byte (one byte) of each transport packet is eliminated and the three-byte value of the counter using the reference clock (27 MHz) is latched at the arrival time of the packet.
The selected program is thus recorded by the digital VTR. A bit rate for each program ordinarily differs in digital transmission depending on the contents of the program. As shown in FIG. 26, bit rates of a high picture quality program, such as a sports program with fast motions or the like, are higher than a program with slower or less motion, such as a music program, a movie program or the like. A bit rate of 15 Mbps is the highest bit rate of the MPEG2. Therefore, it is very wasteful if such programs are recorded at 25 Mbps in the standard mode (SD mode) of the digital VTR.
It is preferable to vary the recording speed according to the bit rate of the program to more efficiently record the signal. Three modes: SD mode, 1/2-SD mode, and 1/4-SD mode can be used. In the 1/2-SD mode, the tape speed is reduced by a factor of two compared to the SD mode. In the 1/4-SD mode, the tape speed is reduced by a factor of four compared to the SD mode. Recording rates and recording times in the respective modes are shown in FIG. 27.
Using FIG. 26 as an example, the 1/4-SD mode is selected for recording/reproducing a program such as the movie program, the music program or the like whose rate is equal to or less than 6.25 Mbps. The 1/2-SD mode is selected for recording/reproducing a program such as a high picture quality program, a sports program or the like whose rate is equal to or less than 12.5 Mbps. The SD mode is selected for recording/reproducing a program having a bit rate exceeding 12.5 Mbps. The bit rate of the selected program is converted to the recording bit rate in each mode by rate converting buffer 102 and a dummy data adding circuit. For instance, by adding dummy data, a string of bits equal to "0", the amount of data recorded to one track in each mode is equalized.
According to the digital VTR shown in FIG. 28, heads A and B are provided on a drum, 180.degree. apart, which rotates at 9000 r.p.m. and a magnetic tape (not shown) is wrapped around the peripheral surface of the drum at a wrap angle that is slightly larger than 180.degree.. The azimuth angles of the gaps of heads A and B are different. Thus, crosstalk from the adjacent tracks can be suppressed upon reproduction and recording can be performed without a guard band.
When the mode is set to the 1/2-SD mode, the signal is recorded on the tape as shown in FIG. 28. In FIG. 28 (and other drawings explained hereinlater) oblique tracks are shown as vertical tracks and oblique lines show azimuth angles for simplicity. On the first scan, head A records the data and head B does not record any data. The same operation holds for the third scan. No data is recorded on the even-numbered scans. Recording is performed by head A or B on designated odd-numbered scans. The tracks in the 1/2-SD mode are formed at the same track pitch as in the SD mode.
As will be understood from FIG. 28, recording is only performed by head A, so that the suppression of crosstalk via azimuth loss is not possible. A similar problem occurs in recording and reproduction in the 1/4-SD mode. When the tape running speed is reduced by an odd factor, such as for a 1/3-SD mode, compared to the tape speed in the SD mode, crosstalk can be suppressed via azimuth loss. However, using odd fraction speed modes limits the range from which recording rates can be selected.